For the operation of the recovery boiler, it is very important that gaseous bodies in the combustion chamber, such as the fed combustion air and the fuel, can be mixed and the fuel can be burned as well as possible so that combustion reactions are mainly over before the flue gases arrive at the upper part of the combustion chamber, between densely located heat transfer surfaces, i.e. superheaters. The fact is that the mixing of gases is prevented or retarded between densely located heat transfer surfaces and the vicinity of surfaces colder than gases stops or retards reactions, though the temperature of the gases otherwise were sufficient for the implementation of the reactions. On account of this, a predetermined retention time in the combustion chamber is necessary, within which time the reacting substances can be evenly mixed. This retention time can generally be caused by increasing the height of the combustion chamber. A problem with big recovery boilers is that even mixing is insufficient within the whole cross-section of the combustion chamber, and it is difficult to adjust the operation conditions of the recovery boiler and the feed of various substances and gases needed for the process. A further problem is that if the flow rate of the gases in the boiler is high, fuel drops try to follow the gas flow and, accordingly, the combustion increases in the upper part of the boiler and causes both corrosion and scaling of the heat transfer surfaces.
With growing effect of the boiler and with accordingly growing size of the combustion chamber, it is necessary to increase the number of cooling surfaces, because the flue gases have to be cooled to a predetermined temperature. From this follows that the bottom area of the boiler and the volume of the combustion chamber increase remarkably when the height grows.
Moreover, the growing cross-section and the accordingly growing height of boilers require higher buildings, and so the construction costs increase in this respect considerably. In addition, because the recovery boiler is suspended from its brackets, a big and heavy char bed on the bottom of the boiler causes a remarkable additional weight load, which still complicates the dimensioning and constructing of the boiler. Further, it is inconvenient and complicated to feed a sufficient amount of air in such a way that the combustion conditions can be made as efficient as possible.